This invention relates generally to magnetic resonance spectroscopy and more particularly, the invention relates to RF pulses for use in spectral editing in MR spectroscopy.
Spectroscopic imaging is a combined spatial/spectral imaging where the goal is to obtain a MR spectrum at each spatial position or to display an image of each chemical shift species at each position. Chemical shift is a subtle frequency shift in a MR signal that is dependent on the chemical environment of a particular compound or metabolite. The chemical shift is a small displacement of resonant frequency due to a shielding dependent on chemical environment and created by the orbital motion of surrounding electrons in response to a main magnetic field, B0.
As described in Cunningham et al., U.S. Pat. No. 6,028,428, there are numerous pulse sequences used in MRI and in NMR spectroscopy. These pulse sequences use at least one, and usually more than one, RF pulse near the Larmor frequency. In addition to the RF excitation pulse mentioned above, such RF excitation pulses may, for example, invert spin magnetization, saturate spin magnetization, stabilize spin magnetization or refocus spin magnetization. When used in combination with a magnetic field gradient, the RF pulses selectively affect spin magnetization over a specific frequency range which corresponds to a specific location within the subject being scanned. Such “selective” RF pulses are thus specified by the degree to which they tip magnetization (“flip angle”) over a range of frequencies.
In U.S. Pat. No. 4,940,940, a method is disclosed for designing RF pulses that will produce a desired flip angle over a specified frequency range. The disclosure of this patent is hereby incorporated by reference. This method, known in the art as the “SLR” method, starts with the desired frequency domain pulse profile (for example, a 90° flip angle over a specified slice thickness/frequency range) and calculates the amplitude and phase of a RF pulse, that when played out over time, will produce the desired result. These calculations involve the approximation of the desired frequency domain pulse profile with two high order polynomials A and B which can then be transformed directly into a RF pulse that is “played out” on an NMR system. The step of producing the polynomials A and B employs a Remez (Parks-McClellan) algorithm that is executed in an iterative process. To calculate the necessary A and B polynomials (hereinafter referred to as the “SLR polynomials”) this iterative process is performed until the desired frequency domain pulse profile is approximated to a specified degree of accuracy.
The use of spectral-spatial EPSE (echo-planar spin-echo) pulses within pulse sequences for MR spectroscopic imaging is an attractive option, as the high bandwidth of the sub-pulses (5–10 kHz) greatly reduces the error associated with chemical-shift misregistration. However, exciting the spectral bandwidth needed to measure a typical set of metabolites (e.g., 300 Hz at 3T), along with an adequate spatial profile, requires high RF amplitude. The amplitude can be brought down into practical range using a RF pulse that excites a profile with non-linear phase but such pulses are inappropriate for J-difference editing of metabolites, such as lactate and GABA.
For editing, it is crucial that the coupled components are flipped simultaneously, and over a short interval. The problem is that a non-linear phase RF pulse affects different resonant frequencies at different times during the RF pulse. These pulses perform poorly for J-difference editing because the coupled components, which have different frequencies, are tipped by the pulse at different times. This causes the spin-echo and J-echo to occur at slightly different times, leading to errors in quantitation.
The present invention provides a new RF pulse design method to enable the use of spectral-spatial RF pulses for J-difference editing in magnetic resonance spectroscopic imaging (MRSI).